This invention relates to a heat pump for recovering usable thermal energy in an industrial process and, in particular, to an industrial heat pump that utilizes an open Rankine cycle.
In many industrial heat pump applications, a heated effluent generated by an industrial process is employed to evaporate a refrigerant moving through a closed loop heat pump circuit. Heat is transferred from the effluence to the working fluid (refrigerant) of the heat pump through means of a heat exchanger generally referred to as an evaporator. The refrigerant is selected so that it will boil at a temperature that is slightly below the process effluent temperature whereby the refrigerant undergoes a phase transformation in the heat exchanger. The refrigerant vapor is then compressed to a higher state and the energy stored in the fluid is passed out of the system by means of a second heat exchanger called a condenser. The refrigerant, which is now mostly in a liquid phase, is throttled to saturation and the cycle is repeated.
Although relatively expensive heat exchangers are used in closed circuit heat pumps, a temperature gradient is established over the heat transfer surfaces of both the evaporator and the condenser which reduces the overall efficiency of the system. By the same token, a relatively large amount of energy is consumed in moving both the process fluid and the refrigerant through the heat exchangers. These parasitic losses also reduce the efficiency of the system.
To reduce compressor work, and thus increaase the COP of the heat pump system, the compressor is sometimes staged and the compressed vapor cooled as it moves between stages. Interstage cooling is usually carried out in a water cooled heat exchanger that consumes relatively large amounts of liquid. Process gases passing through the exchanger experience unwanted pressure losses and, here again, the initial cost of the equipment is relatively high.
In order for an industrial heat pump to be cost effective, its efficiency must be relatively high. When fuel prices are comparatively low, the industrial heat pump does not compete favorably with alternative heating methods. However, with the rapid increase in fuel prices, waste heat recovery systems such as the industrial heat pump is becoming more attractive and any small increase in the overall efficiency of the system helps in attaining the ultimate commercial realization of this type of equipment.